This invention generally relates to control systems for optical amplifiers and is specifically concerned with a method of operating a noise-compensating gain controller for an erbium-doped fiber amplifier that avoids overshoot of a selected gain level during amplification transients.
Erbium-doped fiber amplifiers (EDFAs) are used in optical transmission networks to extend transmission distances and to compensate for losses from various network elements. Such amplifiers typically comprise a pump laser whose output is optically coupled to the input of two or more, serially connected coils of erbium-doped optical fiber. In operation, the output of the pump laser excites the atoms of erbium dopant within the serially connected coils of doped fibers. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output. When such EDFAs are used simply as amplification relay stations along a single, long-distance optical circuit, there is little need for a device to specifically control the amount of gain that the amplifier imparts on the incoming optical signal. However, as optical systems have become more complex, the need for such gain control systems has increased. Such a need may arise, for example, when an optical network is installed around an urban area. Under such circumstances, the distances between the optical amplifiers may be very different. If the EDFAs in the system all have the same amplification capacity, this capacity must be adjusted by way of a gain control device so that the signal strength remains uniform throughout all branches of the network.
In the past, such gain control has typically been achieved by the combination of a digital signal processor in combination with a power regulation circuit that modulates the amount of electrical power applied to the pump laser. The digital signal processor generates a control signal that instructs the power regulation circuit to deliver electrical power to the pump laser at a level consistent with a selected gain set-point. The specific control signal associated with a particular set-point is determined by an empirically derived control algorithm which is programmed into the memory of the signal processor. Hence, when the set-point of the gain controller is selected to be, for example, at 25 decibels (dB), the digital processor generates a control signal that causes the pump laser to amplify the incoming optical signal until the strength of the output corresponds to the amount selected at set-point, i.e., 25 dB.
While such EDFA gain controllers can work well for their intended purpose, the applicant has observed that a significant problem arises when the incoming optical signal is significantly contaminated with a noise component known as amplified spontaneous emission (ASE) in the art. Because such prior art gain controllers amplify the total output to a desired gain level, and because the optical output is nearly always the combination of an amplified signal plus a variable amount of amplified ASE, such controllers under-amplify the signal in direct proportion to the power of the ASE component mixed therewith. Such under-amplification is much worse for low input signals, when the ASE power content may be larger than the signal power. In all cases, the resulting under-amplification of the optical input signal can lead to undesirable non-uniformities in the strength of the signals transmitted through the optical network.
To solve these problems, the applicant has developed a noise-compensating gain controller capable of amplifying the signal component of the optical output to the desired gain level selected by the operator. This controller is described in detail in U.S. application Ser. No. 09/821,926 filed Mar. 30, 2001, and assigned to Corning Incorporated, and generally comprises a gain detecting circuit, a set-point circuit for providing a signal indicative of a selected gain level of the amplifier, and a digital signal processor for adjusting the gain level so that the gain of the signal component of the output is equal to a select gain level. Upon selection of a specific gain level via the set-point circuit, the digital signal processor compares the selected gain level with the actual gain level indicated by the gain detecting circuit, and computes an amplification difference necessary to equalize the actual gain with the selected gain. The processor also computes the amount that the gain will have to be adjusted to bring the signal component of the amplifier output to the selected gain level. This computation is implemented by means of an empirically derived formula that, for every power level of optical input, assigns an associated ASE power level. The processor than proceeds to change the amplification by the computed difference and to adjust this difference to bring the gain of the signal component of the output to the selected gain level.
While such a noise compensating gain controller represents a substantial advance in the prior art, the applicants have observed that an amplification overshoot problem may occur during amplification transients. Such transients typically happen as a result of rapid fluctuations in the power of the optical input, although they could occur from a rapid change in the gain set-point by the system operator. The applicants have determined that such overshoots occur when the digital signal processor does not complete the computation of the amount of noise-compensating adjustment required in the amplification at the same time it completes the computation of the amount of amplification difference necessary to bring the actual gain to the same level as the selected gain. Such differences in computation time occur as a result of the greater complexity of the amplification compensation calculations, and the operating speed limitations of most commercially available processors. Under such circumstances, the processor completes the computation of the amplification difference necessary to bring the actual gain to the same level as the selected gain prior to completing the computation of the adjustment to the gain necessary to bring the gain of only the signal component to the selected gain at a post-transient, steady-state condition of the optical input. The processor then proceeds to change the amplification level in accordance with the completed first computation in combination with a non-final, transient computation of the gain adjustment. Because the non-final computation of the gain adjustment usually yields a value that is significantly higher than the completed computation of the gain adjustment, the initial change of the amplification level is higher than the proper gain adjustment under steady-state conditions, thus resulting in a localized spike or overshoot of the steady state value of the final amount of amplification, as is indicated in FIG. 3B. In the graph of FIG. 3B, the overshoot is approximately 2 dB, and may occur when the total input power of the optical signal changes between xe2x88x9226 and xe2x88x9211 dB, as illustrated in FIG. 3A. Such change could easily occur under normal operating conditions of the amplifier as the result of the adding of channels, which typically occur in time periods of less than 100 micro seconds. The resulting 2 dB overshoot or spike in amplification is highly undesirable in an optical network of interconnected amplifiers, as each amplifier in the network would amplify the overshoot to an even greater height relative to the signal received. The overall effect would be a deterioration in the bit error rate (BER) in the transfer of data.
Clearly, there is a need for a method of operating in noise-compensating gain controller during amplification transients which avoids such spikes of amplification overshoots. Ideally, such a method would not require the addition of any new components or alteration of the connections of the noise-compensating optical gain controller, and could be easily and simply implemented merely by the programming of a relatively simple control algorithm into the digital signal processor.
The invention is a method for operating a noise-compensating gain controller that avoids undesirable overshooting of the gain level during amplification transients. The method is particularly adapted for use in a gain controller having a gain detecting circuit that continuously monitors the gain of the optical input, a set point circuit that provides a signal indicative of a selected gain level of the amplifier, and a digital processor circuit that receives signals from the previously mentioned circuit components, and changes the gain level so that the gain of the entire (signal+noise) output is equal to the selected gain level, and then proceeds to adjust the gain level so that the gain of the signal component only the optical output is equal to the selected gain level.
In the first step of the method, the digital signal processor first determines a difference in the amount of amplification necessary to bring the combination of the signal and noise component forming the optical output to the selected gain level.
In the next step, the digital signal processor determines an adjustment of the difference necessary to bring the signal component of the output to the selected gain level at a post transient, steady-state condition of the optical input.
In the final step of the method, the amplification is changed in accordance with the difference in the amount of amplification determined either prior to or simultaneously with the determined adjustment.
Preferably, the method further includes the step of determining that the power level of the transient is larger than a preselected threshold level prior to implementing the aforementioned steps of determining a difference in the amount of amplification, and determining an adjustment to that difference.
In one embodiment of the method, the computation of the amplification adjustment to compensate for noise is deferred until, amplification is first changed in accordance with the determined difference in the amount of amplification necessary to bring the gain of the total optical output (noise+signal) equal to the selected gain. Then, the amplification adjustment is computed and then added to the amplification.
In another embodiment of the invention, both the difference determination step and the adjustment determination step are implemented simultaneously such that the amplification difference and a post-transient adjustment are determined at substantially the same time. The amplification is then changed simultaneously in accordance with the simultaneously determined amplification difference and amplification adjustment. In one implementation of this alternate embodiment, the processing time for determining the amplification difference is protracted until it is equal to the processing time required to determine the post-transient amplification adjustment. In another implementation of this embodiment, one or more of the lower level tasks that the digital processor normally performs is disabled in order to free up a sufficient amount of processing capacity to complete the computation of the amplification difference and amplification adjustment at a simultaneous time which is sooner than the implementation requiring the protraction of the processing time for the amplification difference determination.
In all instances, undesirable overshoot of a selected gain level is avoided by refraining from adjusting the gain level on the basis of a non-final computation of the noise-compensating adjustment to the gain difference based upon a transient condition of the optical output.